1. Field of the Invention
The present invention generally pertains to optical fibers, and more particularly, to an apparatus and a process for fabricating an optical fiber coated with metal and method therefor.
2. Description of the Related Art
Generally, an optical fiber composed of silica glass and used for communication has a diameter of about 100 .mu.m to 150 .mu.m, and therefore typically is covered with a reinforcement coating in order to prevent abrasion, corrosion, and inadvertent fracture. Theoretically such an optical fiber has a high strength over 14 GPa. If contacted by moisture, however, the fiber is prone to become highly fragile: the moisture tends to enter any minute cracks exposed on the surface of the glass, where it causes the glass to corrode. Because cracks in glass occur at points of intrinsic or externally induced stress, the presence of corrosion at these points inevitably leads to growth of the cracks. As these flaws become more numerous and extend more deeply into the fiber body, the likelihood increases that even a small additional stress, if applied near such a flaw, will cause in the fiber a catastrophic fracture. Optical fibers used for many types of applications therefore require a protective coating to protect the surface of the optical fiber from abrasion, to enhance the tensile and bending strength, and to prevent moisture penetration.
An optical fiber intended for use in field applications, such as for communications transmission lines, typically has its lateral surface covered with a reinforcement coating of a suitable material. Such materials include ceramics and plastic resins, but each of these material types include disadvantages. Ceramics, for example, require expensive equipment to apply them to an optical fiber and tend to have excessive hardness, which can itself damage the fiber. Plastic resins are relatively soft and inexpensive to apply, but the usual varieties (such as acrylic or silicone) may both contain moisture and permit moisture to permeate to the glass surface.
A metal coating has several advantages over ceramics and plastic resins. Metals may be applied with inexpensive machinery, much as plastic resins, but they contain no moisture and provide an impervious moisture barrier. Metal coatings also can provide a high-strength reinforcement to the fiber while having sufficient flexibility to maintain their barrier integrity through substantial bending operations. In addition, a metal-coated optical fiber will retain adequate strength over a substantial usable lifetime, even when subjected to harsh conditions such as high-temperature environments. A metal coating on an optical fiber can also serve, in appropriate situations, as an effective and cost-efficient conductor of electrical signals.
Certain problems also arise from the use of metal coatings. First, some metals tend to react with the glass or cause other undesirable effects when applied in direct contact with a fiber surface. An undercoating of plastic resin could mitigate some of these effects, but the high temperature of molten metals tends to substantially degrade most polymers. Certain metal alloys can be applied in slurry form, at temperatures substantially lower than required for molten metals, but the advantages of a pure elemental metal coating are thereby lost.
Use of molten metal makes the production of a coating with uniform thickness difficult, due to the low viscosity of suitable molten metals. This low viscosity means that rapid movement of the fiber through the molten metal generates turbulence in the metal. Existing methods to prevent this turbulence involve complex additional equipment that increases the cost of coated fiber fabrication. Finally, a metal coating should be substantially cooled before the fiber is wound for storage, but rapid cooling may create flaws such as internal stresses and other undesirable effects in the finished fiber product.
Several approaches to solving the problems accompanying fabrication of metal coatings on optical fibers have met with degree of success. U.S. Pat. No. 4,390,589, issued to Geyling et al., discloses an apparatus and method for coating optical fibers with metal by means of a multiphase alloy bath maintained in a slurry state. Such multiphase alloys have an initial melting temperature substantially below the melting temperatures of the metal constituents of the alloy, and therefore they can be applied successfully as metal coatings over a polymer undercoating. In addition, these alloys have a relatively high effective viscosity that permits immersion of a rapidly-moving fiber without the onset of turbulence. On the other hand, multiphase alloys suitable for this slurry process have relatively low initial melting temperatures that render them unsuitable for coatings on optical fibers to be subjected to extremely high temperature conditions. Moreover, use of an alloy for the metal coating of an optical fiber necessarily sacrifices the advantages provided by a coating of a pure elemental metal.
A metal coating formed from a molten metal bath avoids some of the disadvantages found in a multiphase alloy coating. U.S. Pat. Nos. 4,407,561 and 4,418,984, issued to Wysocki and Wysocki et al., respectively, disclose a process for protecting and reinforcing a silica-glass fiber optical waveguide by coating it with either an elemental metal or an alloy. The process includes applying one or more metal coatings by passing the optical fiber through a molten pool of the metal or alloy, which may have a melting point higher than the softening point of the glass. The product is an optical fiber hermetically sealed by a metal coating. Such a coating may have a melting point well above the highest temperature the optical fiber can tolerate without degradation, and it may be composed of any of several elemental metals.
Passing the fiber through molten metal, though, also creates problems. The fiber must pass through the molten metal sufficiently rapidly that any portion of it remains immersed in the molten metal for only a brief period of time. Otherwise, the coating of solidified metal around the fiber will partially remelt, causing the finished product to have a coating too thin or with breaches. But a high fiber velocity tends to create turbulence in the molten metal, due to the metal's low viscosity. This turbulence. in turn, displaces the fiber from an optimal orientation advantageous for ensuring that the coating has a uniform thickness and therefore leads to a finished product with an irregular outside surface.
Further complications arise from the fact that an optical fiber coating produced from molten metal must cool substantially before the metaled optical fiber can safely be wound and stored. For example, Rand et al., U.S. Pat. No. 4,824,455, discloses a process for producing a polarization-preserving metal coated optical fiber. In this process a fiber composed of core and cladding passes through a bath of molten metal in which a temperature gradient exists. A metal coating results with circular cross-section but greater thickness on one side, which induces an anisotropic stress in the core. Preservation of this configuration in the coating, though, requires the coating to cool at ambient temperature. This slow cooling complicates the fabrication process, particularly when the coating must undergo substantial cooling before the coated fiber can be safely wound.
Gombert et al. have disclosed, in U.S. Pat. No. 4,853,258, a molten-metal optical fiber coating method that avoids the coating thickness variations induced by turbulence in the molten metal yet produces a metal coating sufficiently thick to protect and reinforce the enclosed optical fiber. This method suppresses the onset of turbulence in the molten metal by reducing the hydrostatic pressure on the free surface of the molten metal. Reducing the hydrostatic pressure over the molten metal entails use of a sealed coating vessel that can be substantially evacuated. This in turn requires a vacuum pump and seals at orifices where metal feedstock enters the vessel and where the optical fiber enters and exits the vessel. All of these additional requirements increase the cost of fabricating coated optical fibers with the method.
The formation of an optical fiber and application of a coating on the fiber in one continuous process also raises complications for coating the fiber with metal. Imoto et al., U.S. Pat. No. 4,123,242, discloses an apparatus for drawing an optical fiber from an optical fiber preform while maintaining a uniform fiber diameter by feedback control of the drawing tension. An increase in drawing tension reduces the diameter of the fiber as it is drawn, but it also increases the drawing speed. Thus a typical system for maintaining a uniform fiber diameter involves variations in the fiber drawing speed, but such drawing speed variations will produce variations in a metal coating applied by passing the fiber through a molten metal bath.
On the other hand, U.S. Pat. No. 4,948,406, issued to Kornmann, discloses an apparatus and process for covering an optical fiber with a metal coating having a relatively uniform thickness. The apparatus includes adjustable dies through which the fiber passes before entering and after exiting the molten metal. These dies allow the dimension of the metal-fiber contact region to be varied precisely according to the molten metal temperature, the nominal diameter of the uncoated optical fiber, and the nominal drawing speed. However, this system does not accommodate variations in the drawing speed. Moreover, it requires short fiber immersion times, which in turn requires high drawing speeds that tend to induce turbulence in the molten metal.